There has been tremendous effort in restorative dentistry to investigate the effect of shrinkage and stress of a dental composite on the longevity of a tooth restoration. Theoretically, either lower shrinkage or lower stress should be helpful in minimizing the failure in tooth restoration. However, such a statement might not be true in reality because of the complex nature of tooth restorations which involve not only the restorative material, such as a low shrink or low stress composite, but also involve tooth cavity preparation, adhesive application, composite placement, and curing process/efficiency, and the like. All of these factors are highly dependent on how a clinician understands and masters the dental material and the restoration procedure.
The origin of stress from a composite in adhesive restorations is attributed to the restrained shrinkage, which is a direct result of a curing or polymerization process and is highly dependent on the configuration of the restoration. In addition, the non-homogeneous deformations during functional loading can promote and/or damage the restored tooth, the interface as well as the coherence of the composite or the composite itself. Certainly, the damage from shrinkage or contraction stresses can be reduced by application of an elastic lining at the adhesive interfaces and by slowing the initial curing process.
Unfortunately, it is part of such a complicated process that makes the lack of a convincing clinical study about the correlation between polymerization shrinkage and stress and the longevity of a restored tooth. One primary reason for the difficulty in such a study is caused by the lack of a standard test methodology for effective measurement of shrinkage and/or stress.
There are test methods for polymerization shrinkage as well, which could be classified in two categories: (1) measuring directly by the dimensional change and (2) measuring indirectly by the density change.    1. Archimedes' or buoyancy method: the density of the a material is measured before and after curing by water Pycnometer and a buoyancy balance. Then the difference is calculated to generate the total volume shrinkage at a given measurement time.            Advantages are that the method is simple, easy, quick.        Disadvantages are that the method is rough, inaccurate, no shrinkage kinetic, liquid or sticky or moisture sensitive materials can be tested, porosity in sample demonstrates significant effect on the results.            2. Water or Mercury dilatometer equipped with LVDT (Linear Variable Differential Transducers) allows for the direct measurement of the replacement of any volume during and/or after curing.            Advantages are that the method is accurate and that shrinkage kinetics and absolute volume shrinkage can be measured.        Disadvantage are that the method takes time and may consume materials sensitive, and work with liquid or sticky materials or moisture-sensitive materials is not possible in water dilatometer case. Shrinkage is dependent on the given time, curing intensity and duration.            3. Watts method is where the linear displacement of the bent thin glass or plastic cover caused by shrinkage is registered by LVDT or micrometer during/after curing, and then volume shrinkage is calculated according to a formula with the assumption of uniform shrinking.            Advantages of the method are shrinkage kinetic small sample, and sticky sample are acceptable for measurement.        Disadvantages of the method are that the early stage is less sensitive so it is less accurate, the method is sample size dependent, and also light intensity and curing time dependent.            4. Zurich method is similar to the Watts method, but the movement of a metal plate is registered by an infrared (“IR”) beam.            Advantages of the method are that it is accurate, small samples are acceptable, and sticky samples are acceptable.            5. CCD/Image method includes high resolution CCD (Charged-coupled Device) video image that is recorded and analyzed before and after curing.            Advantages if this method include direct absolute volume shrinkage determination, and that there is no heat effect on any size as long as the sample is fully cured.        Disadvantages of the method include a decrease in accuracy due to the image resolution, and a shrink kinetic defect within sample may be possible.            6. Gas Pycnometer measures direct volume replacement by inert gas that is registered before and after curing.            Advantages of this method are that it is quick, easy, any materials can be measured, long term shrinkage can be measured and temperature does not significantly impact the measurement.        Disadvantages include no shrinkage kinetic, too sensitive due to the extreme penetration capability or helium gas.            7. Strain Gauge method may be considered a stress test method due to its nature in monitoring post-gel polymerization contraction by the change of strain of a known modulus material that is directly contacted with the specimen.            Advantages are that the method is easy and quick.        Disadvantage is that only post polymerization shrinkage is recorded.            8. Micrometer method includes measurement of macroscopic dimensional change that is recorded by a micrometer before and after curing.            Advantages are that the method is easy and quick.        Disadvantages are that the method is inaccurate and too rough.        
In addition, more attention recently has been focused on polymerization (or contraction) stress because there is actually no linear correlation between polymerization shrinkage and polymerization stress within any curable system. The total contraction stress does not depend only on how much the material shrinks it also kinetically depends on the evolving elastic modulus (“stiffness”) of such curable material upon curing. Furthermore, the overall curing stress trapped within in the cured material also depends upon the constrained environment (the shape of the cavity), the established bonding between the cured material and its substitute (the tooth). Contraction stresses from polymerization contraction or polymerization shrinkage in composite restorations is able to deform a restored tooth. This may be reflected as de-bonding, micro-leakage, enamel/dentin cracking, and/or post-operative sensitivity. It also should be pointed out that the contraction stress only becomes a severe issue when the materials are used in well-bonded cases. In other words, only under increasingly constrained conditions, such as heavily bonded posterior restoration, extra attention should be paid to stress.
“Curing Stress” is used herein to refer to the stress developed in an adhesive restoration process instead of “shrinkage stress” or “polymerization stress” for the following reasons.    1) “Curing” or “setting” is commonly used to described the polymerization process involving cross-linking and the polymerization process involved in adhesive restoration is a cross-linking process.    2) Though such a stress is originated from contraction or shrink due to polymerization, not all of polymerization or its contraction (shrink) will contribute to the stress that would cause a clinical problem or sever damage to a restored tooth.    3) In a non-crosslinking (linear or branching) polymerization process, much less contraction (shrink) is converted into stress due to the less restricted nature (high chain mobility) of the formed polymers.    4) In a cross-linking polymerization process more contraction (or shrink) is converted to stress due to the increasing difficulty of chain mobility (highly restricted), which can result in trapped stress within a cured system.    5) Polymerization shrinkage is not necessarily linearly related to the curing stress.    6) In an adhesive restoration cases, additional restriction occurs in the curing composite, that is bonding to the surrounding tooth. The more bonding surface (higher C-factor), the more restriction, which is the reason that low curing stress would matter more in posterior restorations than in an anterior restorations. Of course, reduced curing stress would always be welcomed by any anterior application as well.    7) In posterior restorations with current adhesive composites there is an increased restriction, which is not good for reducing the impact of curing stress, but the curing stress could be reflected in different ways, depending upon its compatibility with (or relation to) the bonding system.            If the rate of development of the trapped stress within the curing composite is faster than the rate of bonding strength to bond composite to the tooth (dentin/enamel), an interfacial gap may be formed. In this case, future tooth cracks might be effectively avoided.        If the rate of composite polymerization is too slow than the bonding between the composite and a tooth (dentin/enamel), then cracks within the cured composite may occur. In this case, future cracks within tooth could also be avoided.        If good cure with the composite and excellent bond between such cured composite and tooth (dentin/enamel) are achieved (for most cases in current adhesive restorations), the impact of the trapped curing stress on the restored tooth will be strengthened because of the increasing restriction. An increase in tooth cracks and sensitivity may result.        
Therefore, to a restored tooth the curing stress is just a dynamic reflection to a delicate balance between molecular contraction (shrinkage) and network formation (modulus) in a well-bonded system through entire curing process. It is believed that the curing stress generated by constrained shrinkage is more harmful clinically than that from an unconstrained shrinkage. In another word, curing stress becomes ISSUE only due to a constrained restoration, in which polymerization shrinkage would be more effectively converted into the “deadly” force. Under less constrained condition (such as anterior application), however, less clinical issue from curing stress would be resulted though same polymerization shrinkage might be involved.
There are many efforts in developing an effective test method to measure contraction stress or polymerization stress as well. For example, U.S. Pat. No. 6,871,550 discloses a method and an apparatus (tensometer) for measuring the characteristics of curing polymers, which utilizes cantilever beam technology to determine characteristics of a polymer during the curing process, including stress-related forces that developed during the polymer curing process. Such a tensometer also provide for controlling and monitoring environmental condition during the curing process. Such an apparatus helps us understand more about polymerization stress, particularly the results suggest that there is not necessarily a linear relationship between shrinkage and stress during the involvement of variable modulus of curing materials, which on the other hand allow developing new resin systems and/or formulated compositions to reduce polymerization stress other than shrinkage approach. However, unlike shrinkage, which has been well pictured by conventional wisdom as dimensional reduction or gap formation, to picture curing stress remain quite challenge.
Obviously, there is strong desire for a way to effectively relate such contraction stress to a day-to-day operation. More specifically there is need to creatively illustrate the complicated phenomena, polymerization stress, in a simple way so as to have better understanding about the possible impact of curing stress. Therefore, the primary objective of present invention is to provide a solution to such a need.